1. Field of the Invention
This invention relates to integrated circuit packaging technology, and more particularly, to a method of achieving solder ball coplanarity on a BGA (Ball Grid Array) integrated circuit package.
2. Description of Related Art
A BGA integrated circuit package is an advanced type of integrated circuit package which is characterized in the provision of a two-dimensional array of solder balls on the bottom surface of the package body for mechanically bonding and electrically coupling the package body to a printed circuit board (PCB). Conventionally, an SMD (Solder Mask Define) method is used for the implantation of solder balls on the bottom surface of a BGA package. Fundamentally, it is strictly required that the implanted solder balls on the bottom surface of the BGA package be highly coplanarized so that the BGA package can be flatly mounted on the printed circuit board; otherwise, it would result in poor jointing of the solder balls with the electrically-conductive parts on the printed circuit board, making the mechanical bonding and electrical coupling of the BGA package unreliable to use. It is common that most BGA package bodies would suffer from warpage from mechanical or thermal stress before performing solder ball implantation.
Many patented methods have been proposed as a solution to the coplanarity problem, such as the U.S. Pat. No. 5,641,946. This patent is characterized in the use of a special SMD process with a solder mask having variably-sized openings to control the collapse of the implanted solder balls to thereby achieve solder ball coplanarity.
The foregoing patented method of U.S. Pat. No. 5,641,946, however, has the following drawbacks. First, the use of reduced mask openings in the SMD process would undesirably result in a poor ball shear and thus a poor solder joint. Second, since the resolution of solder balls defined by the solder mask is low, the use of variably-sized solder mask openings for solder ball height control would be highly complex in procedure, and thus is difficult to carry out.
It is therefore an objective of this invention to provide a new method for achieving solder ball coplanarity on a BGA package, which can achieve solder ball coplanarity without having to use a solder mask having variably-sized openings so that the overall fabrication process would be easy to carry out.
It is another objective of this invention to provide a new method for achieving solder ball coplanarity on a BGA package, which can achieve high solder ball shear for robust solder joint.
In accordance with the foregoing and other objectives, the invention proposes a new BGA package fabrication method that can achieve solder ball coplanarity on warped BGA packages, including concavely-warped and convexly-warped BGA packages. The method of the invention, broadly defined, comprises the following procedural steps: (1) determining in advance a relation of solder ball height against pad size; (2) measuring the warpage degree of the integrated circuit package; (3) preparing a substrate having a front surface for die attachment and a bottom surface for solder ball implantation; (4) forming a plurality of solder-ball pads over the bottom surface of the substrate, each solder-ball pad having a specified size predetermined in accordance with the measured package warpage from said step (2) and the relation of solder ball height against pad size from said step (1); (5) forming a solder mask having a plurality of fixed-size openings over the bottom surface of the substrate, each solder mask opening being aligned with one of the solder-ball pads on the substrate; and (6) performing a solder-ball implantation process to implant a plurality of solder balls through the solder mask openings onto the solder-ball pads on the substrate, the implanted solder balls substantially having coplanarity after reflow over the solder-ball pads.
The foregoing method of the invention is characterized in the provision of the array of variably-sized solder-ball pads over the bottom surface of the substrate, in which each solder-ball pad has a specified size predetermined in accordance with pre-measured package warpage and predetermined relation of solder ball height against pad size. This provision allows the use of a solder mask having fixed-size openings, as contrary to the prior art that uses a solder mask having variably-sized openings, while nevertheless allowing the implanted solder balls to achieve coplanarity and have robust solder joint.